You could have tested for the corona virus, but that would not have helped prevent this - nor is there any treatment to stop corona virus from mutating. I don't even think there is still enough known about it that any vet would have recommended you try to strengthen his immune system after he would have tested positive for the corona virus.Maybe now, looking back, I could have strengthen his immunity earlier. Or checked for the virus...or whatever.
Literally, there is nothing you could have done. So, please don't dwell on the 'what-ifs'. Spend time with your baby and love him all you can.
I don't know that I would ever do it again, but when Tawny was diagnosed with FIP, we did have the vet drain his belly fluid a couple of times to relieve pressure on his abdomen/chest. It did make him feel better, but in all reality - other than to give myself more time to digest to the whole thing - what was I really accomplishing? It was long ago enough that perhaps nowadays vets would discourage such a thing - I don't know.
What is his name? And, should you feel like it, we would love to see a pic of him - totally understand if you do not feel/want to.
Keep talking with us if you want to just vent and help express some of the feelings you are going through.